1. Technical Field
The disclosure described herein generally relates to RF power systems and, more particularly, to an RF power system including a variable frequency RF power source.
2. Description of Related Art
In a plasma processing field such as manufacturing of semiconductors or flat panel displays, an RF power generator supplies RF power to a load to discharge capacitively coupled or inductively coupled plasma into a plasma chamber.
The load is a time-dependent dynamic load including plasma. Due to the dynamic load, there is a need for a method of transferring maximum power to the dynamic load by minimizing a reflection wave reflected from a load between the RF power generator and the dynamic load.
Conventionally, two methods have been used for impedance matching between the RF power generator and the load. One of the methods is that a separate impedance matching network including variable elements is disposed between the RF power generator and the load. The other method is that impedance matching is performed by varying a frequency of the RF power generator.
In the case where a variable element impedance matching network is included, the impedance matching network uses at least two variable reactive elements. The variable reactive element may be a variable capacitor or a variable inductor. Conventionally, the variable reactive element is driven by a motor. Conventionally, a maximum/minimum ratio of the variable reactive element is 10 or greater, which is great enough to allow the variable element impedance matching network to perform impedance matching on a wide range of load impedance. Thus, even when a state of plasma is extremely changed, the variable element impedance matching network may perform impedance matching. However, due to driving speed of the motor, the variable element impedance matching network requires matching time of hundreds of milliseconds to several seconds.
On the other hand, when impedance matching is performed by frequency turning or frequency variation, a conventional frequency variable range is about 10 percent. That is, an impedance range of a load, where impedance matching is possible, is very narrow. Therefore, impedance matching cannot be performed by frequency tuning when a state of plasma is extremely changed. In the meantime, matching time reaching impedance matching is several microseconds (microsec) to several milliseconds (millisec), which is very short.
As one of plasma processing processes, atomic layer deposition (ALD) requires repetition of short process steps. In addition, a through silicon via (TSV) process requires repetition of a deposition process and an etching process. A recent deposition or etching process employs a multi-step recipe in which a process condition is changed while keeping RF power. In order to satisfy such a new process condition, an RF power generator and an impedance matching network must perform impedance matching in tens of milliseconds (millisec) or less. Especially, when pulse plasma processing is used, the impedance matching should be performed within several microseconds to tens of microseconds or less. Accordingly, there is a need for a new impedance matching method in which reflected power is reduced within predetermined range in several microseconds to tens of microseconds and a driving frequency is fixed even though a range of plasma load is wide.